Most modern electronic devices, such as radios and cellular telephones, contain at least one acoustic transducer (e.g. microphone, ringer, speaker, buzzer, etc.). An acoustic transducer is an electrical component that converts electrical signals into sound, or vice-versa. Acoustic transducers are easily susceptible to being physically damaged, so they are often mounted in a protective housing with apertures located over the position of the acoustic transducer. These apertures enable the system to transmit or receive sound signals with minimal acoustic loss, while simultaneously preventing large debris from entering the housing and damaging the acoustic transducer. These apertures, however, do not protect the acoustic transducer from incidental exposure to liquids (e.g., spills, rain, etc.) or fine dust and other particulate. To protect acoustic transducers from contaminants such as these, protective acoustic covers are typically utilized between the acoustic transducers and the housing, as a supplemental barrier to the housing apertures. A protective acoustic cover is simply a material that prevents unwanted contamination (liquid, particulate, or both) from reaching an acoustic transducer. It is desirable for a protective acoustic cover to accomplish this contamination protection while minimizing the overall impact to the acoustic loss of the system.
The acoustic loss of a system (typically measured in decibels) is based on the characteristic elements/components that comprise the system, such as the housing aperture size, the volume of the cavity between the acoustic transducer and the protective acoustic cover, etc. The impact each element has on the overall acoustic loss of the system, independent of its area, can be determined individually by calculation or test; and this is called specific acoustic impedance.
For most acoustic systems, the ideal protective acoustic cover would have a specific acoustic impedance value as small as possible. In some cases, however, the acoustic system (minus the protective acoustic cover material) may contain sharp resonances at certain frequencies. In this case, a protective acoustic cover with a higher level of acoustic impedance can be effective at dampening the system resonances and ultimately flatten the spectrum for improved sound quality.
Specific acoustic impedance can be measured in Rayls (MKS), and is composed of two terms: specific acoustic resistance and specific acoustic reactance. Specific acoustic resistance affects the specific acoustic impedance in a uniform manner across the frequency spectrum, and is related to viscous losses as air particles pass through the pores of the protective acoustic cover material. These viscous losses are created by either friction of the air particle on the pore walls and/or a less direct air particle path (i.e. tortuous). Specific acoustic reactance, however, tends to affect the specific acoustic impedance in a more frequency-dependent manner, and is related to the movement/vibration of the protective acoustic cover material in use. Because it has a non-uniform behavior with frequency, materials that are highly reactive are typically not selected for use as a protective acoustic cover, unless the application requires high environmental protection.
As a general rule, the larger the pore size in a protective acoustic cover material (all else being equal), the lower the resulting specific acoustic resistance and the lower the level of liquid and particulate protection. Also generally speaking, the thinner the protective acoustic cover material, the lower the specific acoustic resistance, as well. This is because, as the material becomes thinner, lower viscous losses associated with air particles passing through the pores result. Non-porous materials or ones with very tight pore structures, however, tend to transmit sound via mechanical vibration of the material (i.e. reactance), as opposed to physically passing air particles through the pores. Since vibration is required to transmit sound in this case, materials with high flexibility, low mass and less thickness are desired, in order to minimize specific acoustic reactance. These thin, low mass materials, however, can be more delicate, less durable, and more difficult to handle during fabrication and subsequent installation into an electronic device, so very low reactance may not be achievable in practice. The fact that the properties of acoustic resistance, acoustic reactance, durability, manufacturability, and contamination protection are often competing have made it difficult to develop protective acoustic materials that simultaneously meet aggressive acoustic and liquid and particulate protection targets. This has resulted in two major categories of protective acoustic covers: ones that can give high liquid and particulate protection, but with a relatively high specific acoustic impedance (usually dominated by reactance); and ones that offer low specific acoustic impedance, but with an accompanying low level of liquid and particulate protection.
There are several different materials used in the construction of typical protective acoustic covers in use today. Many prior art protective acoustic covers are composed of a porous material constructed of synthetic or natural fibers, formed into either a woven or non-woven pattern. Other protective acoustic cover materials, such as microporous PTFE membranes, contain a network of interconnected nodes and fibrils. Finally, for very harsh or demanding environmental applications, some protective acoustic cover materials are composed entirely of non-porous films, such as polyurethane, Mylar®, etc.
A general description of prior art patents adhering to the above-described scientific principles follows.
U.S. Pat. No. 4,949,386, entitled “Speaker System”, teaches a protective acoustic cover comprising in part, a laminated two-layer construction defined by a polyester woven or non-woven material and a microporous polytetrafluoroethylene (“PTFE”) membrane. The hydrophobic property of the microporous PTFE membrane prevents liquid from passing through the environmental barrier system. However, although this laminated covering system may be effective in preventing liquid entry into an electronic device, the lamination results in an excessively high specific acoustic impedance (dominated by reactance) which is unacceptable in modern communication electronics where sound quality is a critical requirement.
U.S. Pat. No. 4,987,597 entitled “Apparatus For Closing Openings Of A Hearing Aid Or An Ear Adapter For Hearing Aids” teaches the use of a microporous PTFE membrane as a protective acoustic cover. The membrane effectively restricts liquid passage through the membrane but also results in a high specific acoustic impedance. Additionally, the patent fails to specifically teach the material parameters of the membrane that are required in order to achieve low specific acoustic impedance, although it does generally describe the parameters in terms of porosity and air permeability.
U.S. Pat. No. 5,420,570 entitled “Manually Actuable Wrist Alarm Having A High-Intensity Sonic Alarm Signal” teaches the use of a non-porous film as a protective acoustic cover. As previously discussed, although a non-porous film can provide excellent liquid protection, such a non-porous film suffers from extremely high specific acoustic impedance, which is dominated by reactance. This can produce sound that is excessively muffled and distorted. The high specific acoustic reactance results from the relatively high mass and stiffness associated with typical non-porous films.
U.S. Pat. No. 4,071,040, entitled “Water-Proof Air Pressure Equalizing Valve,” teaches the disposition of a thin microporous membrane between two sintered stainless steel disks. Although such a construction may have been effective for its intended use in rugged military-type field telephone sets, it is not desirable for use in modern communication electronic devices because the reactance is extremely high. This is because the two stainless steel disks physically constrain the membrane, limiting its ability to vibrate. Additionally, sintered metal disks are relatively thick and heavy and are thus impractical for lightweight, handheld portable electronic devices.
To overcome some of the shortcomings described above with respect to the '386, '597, '570 and '040 patents, U.S. Pat. No. 5,828,012, entitled “Protective Cover Assembly Having Enhanced Acoustical Characteristics” teaches a protective acoustic cover assembly comprising a membrane that is bonded to a porous support layer in a ring-like pattern. The construction results in an inner, unbonded region surrounded by an outer, bonded region. In this configuration, the membrane layer and the support layer are free to independently vibrate in response to acoustic energy passing therethrough, thereby minimizing the specific acoustic reactance over a completely laminated structure. However, although this construction reduces the reactance of the laminate comparatively, the degree of specific acoustic reactance still remains quite high.
To increase the simplicity, robustness, and improve the liquid protection of the construction described above with respect to the '012 patent, U.S. Pat. No. 6,512,834 entitled “Protective Acoustic Cover Assembly” teaches a protective acoustic cover assembly that eliminates the need for a porous support layer. While this invention provides both improved water intrusion performance and acoustics over the '012 construction, the acoustic reactance still dominates the acoustic impedance.
Although the prior art mentioned above primarily discusses highly reactive materials, most commercially available protective cover materials are typically resistive. Examples of such resistive materials are a polyester woven material with the tradename SAATIFIL ACOUSTEX™ by SaatiTech, a division of the Saati Group, Inc. and nonwoven materials from Freudenberg Nonwovens NA and W. L. Gore & Associates, Inc. As mentioned previously, these materials can have a high specific acoustic resistance, which can be influenced by either their tortuous particle path and/or their increased material thickness. These physical material properties create higher viscous losses associated with the air particles passing through the pores. Because highly resistive materials are often highly undesirable in many applications, materials of this type can be produced with lower specific acoustic resistance, but this is usually accomplished by increasing the pore size of the material. This results in a decrease in the level of liquid and particulate protection.
Because the consumer market desires the use of handheld electronic devices in increasingly harsh environments while simultaneously expecting high reliability and sound quality, the demand for durable, more contamination-resistant and less resistive/reactive protective acoustic cover materials has increased remarkably. Therefore, there exists an unmet need to have a protective acoustic cover with low acoustic resistance, no measurable acoustic reactance, and a high level of water and particulate protection. The acoustic cover should also be durable, and sufficiently rigid to facilitate the use of quick and accurate installation methods. It would also be highly desirable for the protective cover material to offer additional properties and benefits such as: electrical conductivity for EMI shielding, grounding and ESD protection, high temperature and chemical resistance, and compatibility with insert-molding or heat-staking processes to simplify installation into a housing.
The foregoing illustrates limitations known to exist in present protective acoustic cover systems for electronic communication devices. Thus, it is apparent that it would be advantageous to provide an improved protective system to overcome one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.